Abstract
Tardigrades represent an invertebrate phylum with no circulatory or respiratory system. Their body cavity is filled with free storage cells of the coelomocyte-type, which are responsible for important physiological functions. We report a study comparing the ultrastructure of storage cells in anhydrobiotic and hydrated specimens of the eutardigrade Richtersius coronifer. We also analysed the effect of temperature stress on storage cell structure. Firstly, we verified two types of ultrastructurally different storage cells, which differ in cellular organelle complexity, amount and content of reserve material and connection to oogenetic stage. Type I cells were found to differ ultrastructurally depending on the oogenetic stage of the animal. The main function of these cells is energy storage. Storage cells of Type I were also observed in the single male that was found among the analysed specimens. The second cell type, Type II, found only in females, represents young undifferentiated cells, possibly stem cells. The two types of cells also differ with respect to the presence of nucleolar vacuoles, which are related to oogenetic stages and to changes in nucleolic activity during oogenesis. Secondly, this study revealed that storage cells are not ultrastructurally affected by six months of desiccation or by heating following this desiccation period. However, heating of the desiccated animals (tuns) tended to reduce animal survival, indicating that long-term desiccation makes these animals more vulnerable to heat stress. We confirmed the degradative pathways during the rehydration process after desiccation and heat stress. Our study is the first to document two ultrastructurally different types of storage cells in tardigrades and reveals new perspectives for further studies of tardigrade storage cells.
Highlights
Tardigrades represent an invertebrate phylum with many species that have evolved adaptations to survive extreme levels of dehydration and freezing [1, 2, 3, 4, 5, 6]
Declines in storage cell size connected with a period of anhydrobiosis have been reported (Richtersius coronifer (Richters, 1903) [13]; Milnesium tardigradum (Doyère, 1840) [15])
We examined if storage cell structure was affected by heat stress
Summary
Tardigrades represent an invertebrate phylum with many species that have evolved adaptations to survive extreme levels of dehydration and freezing [1, 2, 3, 4, 5, 6]. Tardigrades do not possess circulatory or respiratory systems, but their body cavity is filled with storage (or body cavity) cells, which float freely in the body cavity lymph [7, 8] or sometimes adhere to the basement membrane of other tissues [7] These storage cells are responsible for important physiological functions, primarily nutrient transport and storage of mainly lipids and polysaccharides and pigments such as carotenes [9, 10]. Most studies on heat tolerance in desiccated tardigrades have used short exposure times (1 h), but Rebecchi et al [26] exposed anhydrobiotic tardigrades of the species Paramacrobiotus richtersi (Murray, 1911) to 37 ̊C at 30–40% RH for up to 21 days, with no effect on survival. We examined if storage cell structure was affected by heat stress
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